manifestation of gaseous pressure that its extraordinary character is apt to be overlooked on accounts of its familiarity. Let us, therefore, consider the matter carefully, and perhaps the simplest method of doing so will be to think of a vertical cylinder with a piston that fits closely, but can move freely. Beneath this piston gas is supposed to be present, and we may imagine the pressure to be so adjusted that the piston shall be ascending in consequence of the pressure exerted by the gas. The question to be examined relates to the force by which this upward movement of the piston is caused. At first it might be urged that the force is simply due to elasticity, and of course this is true, though it is far from providing the required explanation of the difficulty. Let us look a little closer into the matter and see if we cannot ascertain what may be the physical character of this so-called elasticity. We have seen that gas is composed of an innumerable host of molecules, and therefore it must be in some way owing to the action of the molecules that the piston is compelled to ascend in opposition to gravitation.
It is necessary to believe that what elevates the piston is nothing more or less than the hammering of the little gaseous molecules underneath. If the gas be compressed into half its volume then the distances of the molecules are lessened, and the number of blows that the piston receives in a given time is doubled, so that the force with which the piston is pushed upwards is also doubled. This accounts for that fundamental property of the gas which declares that when the temperature remains unaltered the pressure varies inversely with the volume. The effect of heat in increasing the pressure of a gas can be sim-
